add warning for projects with manually specified -Werror
add warning for projects with manually specified -Werror

Point people to the "werror" builtin option.

mesonbuild/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,pexip/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,pexip/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,pexip/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson